Probably my biggest issue is trying track long running thread changes. The first time someone posts in a thread that topic gets put in the "Topics I've participated in list." Eventually, that entry gets bumped off the list by new posts. It would be nice if this list better reflected recent activity, and not just what topic I'm new to, but what I've actually been active in. There are a few long running threads I keep an eye on, and it's much harder to follow an old topic even if you are actively participating in it. Those are very handy links, but I often find they are woefully out of sync with what I'm actually doing. Pinning selected topics would be be another way. I really don't care how to track old topics, just that there is a supplied method that works.
You can currently mark threads as favorites, putting them in a list right above "Topics I've Participated in" on the Community landing page. Despite this, I would agree that the list of topics you've participated in could still use a more intelligent ordering.
Count me in on search being terrible. Its completely useless as is.
Full access to at least my own post history would be great. I don't personally care about seeing other users' history, but I can see it being useful as kind of a background check for giveaways and such.
I'm not a fan of the way writing a post opens a new window.
I'd like to see consistency on whether games in a series are merged into one forum or not. Most are merged, but there's a few outliers. I did see the post describing the issues with this, so it may not be worth the time. I'd still like to see it.
Whenever you get around to this overhaul, I'd like some things to stay the same. I'm a big fan of the simple appearance of the forums. No flashy themes, no garish banners, no egregious abuse of signatures (because they aren't supported). In particular, if you do decide to add support for signatures, please do not allow images in them or at the very least make very restrictive size limits. Very little is more aggravating than browsing a forum with screen-long signatures with flashing gifs and whatnot.